1. Field of the Invention
Embodiments of the invention generally provide a substrate support utilized in flat panel substrate processing.
2. Description of the Related Art
Liquid crystal displays or flat panel displays (FPD) are commonly used for active matrix displays such as computer and television monitors, personal digital assistants (PDAs), and cell phones, as well as solar cells and the like. Generally, a flat panel display comprises two glass plates having a layer of liquid crystal material sandwiched therebetween. At least one of the glass plates includes at least one conductive film disposed thereon that is coupled to a power supply. Power supplied to the conductive film from the power supply changes the orientation of the crystal material, creating a pattern such as texts or graphics on the flat panel displays. Substrates utilized for flat panel fabrication are large in size, often exceeding 300 mm×400 mm, and are envisioned up to and beyond 4 square meters in surface area. Correspondingly, the substrate supports utilized to process large area substrates are proportionately large to accommodate the large surface area of the substrate.
Plasma enhanced chemical vapor deposition (PECVD) is frequently employed in flat panel display fabrication to deposit thin film on a substrate. PECVD is generally accomplished by introducing a precursor gas into a vacuum process chamber to be energized (e.g., excited) into a plasma. FIG. 1 is a schematic cross-sectional view of a CVD process chamber 2 having a support plate 18 and a susceptor 22 disposed therein to support a substrate 12 (not to scale). Reactive precursor gases flowing into a diffuser plate 16 through a gas inlet 14 near the top of the process chamber 2 are excited to form a layer of material on the surface of the substrate 12 that is positioned on a temperature controlled substrate support or susceptor 22. An opening 10 disposed in the sidewall 8 allows a robot (not shown) to deliver and retrieve the substrate 12 to and from the process chamber 2. A support plate 18 coupled to a support shaft 20 to support the susceptor 22 is typically made of a single rectangular plate of ceramic material, such as aluminum oxide, and closely covers the area of the susceptor 22. The susceptor 22 for a CVD chamber historically has been made of a single rectangular plate of aluminum and is typically heated by an embedded heater (not shown) with thermocouples and energy supplied from a power source 24. The heater can also be positioned to the back side of the susceptor 22 or clamped onto the susceptor 22 by a clamp plate.
Generally, the substrate support of the process chamber 2 may be heated from room temperature to a high temperature of less than 500° C., and the susceptor 22 can deflect and “droop” without adequate support. The ceramic material of the support plate 18 has been used to support the susceptor 22 made of ductile aluminum. However, ceramic is a relatively poor thermal conductor and, thus, demonstrates a temperature gradient between a hotter upper surface of the support plate 18 that contacts the heated susceptor and a cooler lower surface of the support plate 18 and as a result, the support plate 18 deflects downwardly at its outer perimeter. A substrate supported by the susceptor is prone to conform to the susceptor and, thus, also deflects. As a result, the vertical spacing between the diffuser plate 16 and the substrate 12 varies between a central portion of the substrate 12 having a distance 34 from the diffuser plate 16. A greater distance 36 resulting from large degree of deflection is located near it perimeter. The difference in the vertical spacing (i.e., substrate deflection distance) greatly decreases uniformity of the deposited films disposed on the large area substrate.
In addition, after striking a plasma inside a PEPVD chamber, the energy from the plasma also creates heat directed toward the substrate and the substrate support, e.g., the susceptor. Therefore, there is a problem of a temporal temperature increase or spike (e.g., about 30-50° C. increase or 20%-30% temperature increase from 150° C.) for the processing substrate disposed on the susceptor. Such drastic temperature variation needs to be controlled in order to maintain a constant temperature on the substrate being processed. In addition, cooling of the susceptor of a process chamber is also needed after processing and during remote plasma cleaning, RF assist cooling, and/or chamber part cleaning and maintenance. However, most PECVD chambers either do not have any cooling design inside the susceptor (i.e., slowly cool down by itself to room temperature) or use cooling mechanisms surrounding the back side of the substrate, not in the susceptor. These prior art designs present difficulties to maintain a constant process temperature for a large area substrate, often leading to local temperature variations over the large surface of the substrate. As a result, variations in film thickness, often manifesting as spots of thinner film thickness, have been observed, which is detrimental to the next generation of flat panel or solar cell devices.
Therefore, there is a need for an improved method and apparatus to control the temperature of a substrate support constantly to a desired range.